Field of the Invention
The present invention relates to an ophthalmic apparatus, which is configured to acquire an image of a fundus of an eye to be inspected, for example. More specifically, the present invention relates to an ophthalmic apparatus capable of acquiring a small area image of a minute region of the eye to be inspected at a high resolution, and of acquiring a large area image of a larger area of the eye to be inspected at a low resolution.
Description of the Related Art
There has been known an aberration compensation technology in which aberrations of reflected light of light projected on a fundus of an eye to be inspected are sensed by a wavefront sensor arranged at a position conjugate to a pupil of the eye to be inspected, and in which the aberrations are compensated by an aberration compensation device. There has also been performed an investigation in which the aberration compensation technology is utilized to acquire an image of a minute region of the fundus at a high resolution, and in which information on a shape and a density of photoreceptors, a flow of blood cells, and the like is used for diagnosis. Moreover, at the time of acquisition of the image of the minute region, a fundus image acquired of a large area is used to select a region on which the high-resolution image is to be acquired. Therefore, there has been known the structure in which, separately from a fundus image acquisition portion configured to acquire the image of a small region of the fundus at the high resolution, a large area fundus image acquisition portion configured to acquire the image of a large region on the fundus is provided.
In the above-mentioned structure, a dichroic mirror is commonly used to divide light from the fundus into two optical paths leading to the optical system configured to acquire the small area image and the optical system configured to acquire the large area image, which have been described above. In this case, a wavelength of measuring light used in the optical system configured to acquire the small area image is set to be different from a wavelength of measuring light used in the optical system configured to acquire the large area image. Then, the measuring light obtained from each of light sources having different wavelengths of the respective optical systems is irradiated on the fundus, and return light from the fundus is divided by the dichroic mirror (see Japanese Patent Application Laid-Open No. 2014-79517). With this method, acquisition of the small area image and the large area image of the fundus may be realized at the same time.
However, the image acquisition apparatus using the optical systems described in Japanese Patent Application Laid-Open No. 2014-79517 has a problem in that ghost is caused. In order to adapt to a diopter of an individual eye to be inspected, the image acquisition apparatus includes a focusing optical system configured to adjust the diopter. In order to adapt to an eye to be inspected on a myopia side, when an attempt is made to focus the image acquisition apparatus on the fundus, a conjugate point of the fundus is brought closer to the eye side as compared to an emmetropic eye.
Here, in FIG. 8, a correspondence between a diopter and a distance from the eye to the conjugate point of the fundus is illustrated. For example, when a degree of myopia of the eye to be inspected is −12 diopters (D), the conjugate point is located 86 mm (=1,000/12) from the eye. Meanwhile, when the diopter of the eye to be inspected is changed from the myopia side to a hyperopia side, the conjugate point of the fundus is shifted to the side of the light source of the measuring light along with the change.
Therefore, when the structure in which a lens is arranged on the light source side of the conjugate point is adopted as in Japanese Patent Application Laid-Open No. 2014-79517, the return light, that is, the ghost is inevitably caused at a lens surface. Further, when the conjugate point coincides with the lens surface, especially strong ghost is caused. When such ghost enters an avalanche photodiode (APD) sensor, image quality is significantly deteriorated. Moreover, when the ghost enters the wavefront sensor, the sensor malfunctions due to the ghost, and cannot perform appropriate wavefront compensation.
In order to solve the above-mentioned problem, it can be contemplated to avoid causing the ghost by reducing a surface reflectance of the lens. Meanwhile, a reflectance of the fundus, of which the image is to be acquired, is extremely low, and hence an intensity of the light from the fundus becomes lower. To address this problem, the reflectance of the lens surface at least needs to be 0.05% or lower. However, in mass production, stably reducing the reflectance of the lens surface to 0.05% or lower is difficult in terms of forming a thin film on the lens surface, and hence is not practical at present.
It can also be contemplated to avoid ghost by arranging the lens as close to the eye side as possible with respect to the conjugate point of the fundus. However, in that case, a distance (working distance) between the eye to be inspected and the dichroic mirror is reduced. When the working distance is small, there may arise a problem in that the apparatus and the nose interfere with each other, for example.
It can further be contemplated to interchange the arrangement of the dichroic mirror and the lens to obtain the structure in which the lens and the dichroic mirror are arranged in the stated order from the eye side. However, in this structure, the lens is shared for both optical paths of the optical system configured to acquire the small area image and the optical system configured to acquire the large area image. In this case, particularly in the optical system configured to acquire the small area image, the wavefront sensor, which has high sensitivity to ghost, may cause malfunctions, and hence it becomes difficult to acquire the image with high resolution.